1. Field of the Invention
This invention relates generally to a switching motor control and, more particularly, it relates to a switching motor control for a D.C. motor utilizing a microcontroller to replace at least a portion of the analog circuitry in the motor control.
2. Description of the Prior Art
The availability of faster and more proficient semiconductors having higher voltage capabilities have guided designers away from using step-down transformers in power supplies in order to reduce the cost and size for D.C. motor controls. More motor controls are being designed to operate off-line, i.e., the motor controls immediately rectify and filter the 120 volt AC line and produce a 170 volt DC rail.
In the prior art, the pulse width modulation (PWM) circuitry utilized in the motor-controllers has required op-amps, comparator, and delay circuits to generate the error voltages of the control loops and the PWM waveforms to drive the power stage. Still, more op-amps are used to generate the I.R. (internal resistance) compensation which is so common in D.C. motor control. Achieving noise rejection to protect control signals from the transients generated by the switching transistors has been difficult and has required more components and op-amps in the form of synchronous and integrating filters.
With off-line switching motor controls, it is not unusual to drive a 120 volt DC motor. Since the power supply can be 170 volt DC with a typical line of 120 volt AC, the maximum torque that the motor could deliver to the load is quite high. The user input to these motor controls is typically in the form of a DC voltage from a potentiometer or a D/A converter. If this voltage is corrupted and becomes inappropriately high, the motor can seriously damage the load and the user. Corrupted voltage can occur in a number of different ways ranging from loss of continuity to the input signal to a total failure of the output power handling device.
The output power handling stage of the prior art motor controllers is typically a single transistor and a free-wheeling diode. This combination provides PWM at a very low cost and high efficiency. If, however, the power transistor should become shorted (not an unusual failure mode) when the power is applied, the full 170 volt DC supply would be applied across the motor and the motor will accelerate at a phenomenal rate. Such acceleration could be very dangerous and potentially damaging to the load and the operator.
Over-temperature protection has previously required the addition of a temperature measuring device in the form of circuitry to buffer the signal and circuitry to cause the protection. The addition of a temperature measuring device adds cost, complexity, and parts to the motor controllers.
The switching motor control of the present invention solves many of the common motor control problems in the prior art by the implementation of a small-inexpensive microcontroller which consolidates most of the functionality which were previously to be accomplished in discrete circuitry.